With today's flourishing global mobile communication development, operators are continuously aiming at reducing total cost of ownership (TCO) and identifying cost-effective methods of expanding mobile networks. This is necessary in order to cope with the rapidly growing needs of users. In order to lower the TCO, a base station may be equipped with multi carrier technology i.e. the base station may use a plurality of carriers, or carrier frequencies, within a transmitter. Such a base station may be called a multi carrier base station. This multi carrier base station can be a multi carrier in a transmitter frequency band, or multi carrier in different transmitter frequency bands. Also, the multi carriers can be for a single radio access technology (RAT) or for different RATs. A multi carrier base station needs to support high output power and a wide range of bandwidth to provide both coverage and capacity to the cell.
At the same time, a transmitter is normally non-linear, which means that the output of the transmitter is a non-linear function of the input. A non-linear multi-carrier transmitter will produce additional spectral components at its output, at frequencies differing from the transmitting frequencies. These spectral components are referred to as intermodulation products (IM products), and have a specific frequency relation to the transmitted signals. Such products may be active IM products and passive IM products. Since the IM products are spread out across the frequency spectrum, outside the transmitting frequencies, there is a risk that some of the IM products end up in the frequency spectrum of the receiver of the base station, thereby causing distortion to a signal received at the receiver. Since modern multi-carrier base stations support high transmit power and have a plurality of carriers arranged over a widespread bandwidth, the IM products may cause considerable distortion to a received signal.
The non-linearity of the power amplifier of the transmitter can be regarded as the main source of intermodulation distortion, i.e. distortion caused by IM products. There is today technology, such as digital or analog pre-distortion arrangements to take care of distortion caused by IM products originating from the power amplifier. Further IM products originating from transmitter components may be attenuated by a transmitter band pass filter arranged before the signal to be transmitted is fed to the antenna.
Although, there are also physical components arranged after the transmitter band pass filter in a base station, which components may cause IM products. Examples of these physical components include antenna(s), cables connecting the transmitter and the receiver to the antenna(s), connectors for connecting the antenna(s) and the transmitter and the receiver to the cables, and the transmitter band pass filter itself. Even though IM products caused by such physical components are much lower than the IM products caused by e.g. the power amplifier, they may cause distortion to a received signal when the IM products occur in the frequency spectrum employed by the receiver, especially if the received signal is weak. Since these IM products originate from components arranged after the transmitter band pass filter, they cannot be filtered out by the transmitter band pass filter.